While there was no indication of a fire, plant personnel are "taking conservative actions" and putting the unit out of service to investigate what happened, a statement from utility said. One of the Prairie Island nuclear power plant's two units in Red Wing is being shut down Thursday in response to what Xcel Energy is calling an "unusual event." According to the utility, a fire alarm went off overnight in the Unit 2 containment building. While there was no indication of a fire, plant personnel are "taking conservative actions" and putting the unit out of service to investigate what happened, a statement from the Minneapolis-based utility said. "There is no release of radioactive materials as a result of this event, " which was declared shortly after 4 a.m., the statement added. Unit 1 is operating normally, said company spokesman Tom Hoen. The incident has not affected the utility's ability to generate electricity, Hoen said. A notification of an "unusual event" is the lowest of four emergency classifications established by the U.S. Nuclear Regulatory Commission. The notification indicates a potential reduction in the level of safety at the plant but no threat to public safety, the utility explained. Hoen said he was expecting more information about the shutdown later Thursday morning. In December, Unit 1 was taken out of service to allow workers to replace a reactor coolant pump seal during an outage. Xcel Energy has one other nuclear power plant in Minnesota, a one-unit facility in Monticello, northwest of the Twin Cities.